www.1000projects.comwww.fullinterview.comwww.chetanasprojects.comAbstract:

For over a century electrical trans- -

mission systems have been based

mainly on overhead transmission

lines (OHL). The main reason for

this has been the cost advantage

when compared to high voltage

underground transmission. Recent

studies suggest the cost premium of

underground transmission is in the

range of 5 – 15 times the traditional

overhead transmission alternative.

But this comparison is already dated.

Two main factors are affecting the

paradigm:

Environmental

restrictions are

increasing the costs

and implementation

time for overhead

transmission.

Technological

development

significantly reduces

the cost of

underground

transmission.

What makes underground

HVDC cable preferable?

There are several reasons why

underground HVDC cables have a

better environmental profile than

overhead HVAC lines.

LAND USE:

An HVDC cable uses significantly

less land than an overhead HVAC

line. The right-of-way for a 400 kV

OHL

can be a 60 m wide strip where no

buildings/high trees are allowed

whereas an underground DC cable

needs at most a 4 m wide inspection

road on top of it. For AC OHL the

amount of land required for a

transmission is 2,400 hectares (1

hectare = 10,000 m2). However only

160 hectares are required forDC

cable (< 6 percent).

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AUDIBLE NOISE:

Restrictions on land use stretch

beyond the immediate right-of-way.

Audible noise from transmission

line corona – most noticeable when

conductors are wet in foggy

weather conditions – might restrict

buildings close to OHL. The width

of this “noise corridor” depends on

local noise ordinance as well as on

the design and voltage of the line.

Noise objections from neighbors

make it more difficult to obtain

permits. An underground DC cable

naturally has no audible noise

emission.

ELECTROMAGNETIC FIELD:

Magnetic and electrical fields can

also restrict the use of land close to

an OHL. In several countries a

precautionary policy vis-à-vis

magnetic fields is in force. The

Swedish National

Electrical Safety Board and the

Dutch Ministry of Housing and

Environment both suggest a 0.4 μT

safety

level for 50 Hz magnetic fields from

transmission lines. This level

corresponds to field levels normally

encountered in city environments

today. In contrast to an AC line, the

field for a DC cable is static (non-

radiant). Applying the same

precautionary policy as for AC

would not call for the provision of

any “EMF corridor” around an

underground DC cable. The field

immediately above the cable is far

less than the earth’s natural

magnetic field.

Right-of-way as a loss of CO2 sink

Growing forests are considered

CO2 sinks because trees convert

carbon dioxide from the

atmosphere into carbon stored in www.1000projects.comwww.fullinterview.comwww.chetanasprojects.com

www.1000projects.comwww.fullinterview.comwww.chetanasprojects.comthe form of wood and organic soil

matter. A forest can absorb 9.2 tons

of CO2 per hectare per year.

Building a 400 km, 400 kV

overhead transmission line through

an area that is 75 percent forest

represents a loss of a carbon sink of

16,780 tons of CO2 per year.

HVDC Light technology was

introduced in 1997 with a small

test installation

of 3 MW. Since then both cables

and converters have progressed

dramatically

in both size and performance.

MATERIAL USE:

The material intensity of an AC OHL

is higher than a DC cable. The

statistical material use per meter of UNDER GROUNDMATERIAL D.C A.C OHL Aluminium 2.1kg 3.3 kg Copper 1.4 kgPVC 2.3 kgPEX 6.1 kgSteel 100.0kg Ceramics 0.3kg Concrete 376.3kg

transmission is compared in below

table Using life cycle assessment

(LCA) to analyze the “cradle to

grave” material impact, the DC

cable has an environmental impact

of 64.5 kg of CO2- equivalents per

meter and the AC OHL has an

impact of 365.4 kg of CO2-

equivalents per meter. In other

words, the material used in the DC

cable has only 17.6 percent the

environmental impact of the AC

OHL.

AESTHETICS – PROPERTY

VALUE:

Several studies have shown that

property values are reduced close

to OHL. For example, a study

carried out in the United Kingdom

showed the value of detached

properties a distance of 100m from

OHL were 38 percent lower than

comparable properties. A Finnish

study showed that the reduction is

proportional to the distance from

the line . Assuming that every 500

m along the 400 km line there is:

One property 500 m from the OHL

(with 8 percent value reduction).

Two properties 1000 m from the

OHL (with 4 percent value

reduction). Three properties 2000

m from the OHL (with 2 percent

value reduction). If an average

property is valued at $ 150,000, the

reduction in property value along www.1000projects.comwww.fullinterview.comwww.chetanasprojects.com

www.1000projects.comwww.fullinterview.comwww.chetanasprojects.comthe 400 km OHL then amounts to a

staggering $ 25 million.

ELECTRICAL LOSSES

When HVDC Light underground

transmission is used inside an AC-

grid, the transmission system can

be operated in a more optimal way

leading to lower electrical losses.

The to the loss reduction of the AC

grid,i.e., the HVDC line is

considered to transmit electricity

“without” losses. The more efficient

operation of a transmission system

with HVDC can be attributed to

two causes:

THE average higher voltage level in

the AC grid and the reduction of

reactive power flows.

For example,

On a 350 MW transmission (50

percent utilization) there are no

HVDC losses whereas HVAC losses

amount to 5 percent. This means

the operator has 76,650 MWh more

electricity to sell each year with an

HVDC connection. The overall

electrical losses1) can be translated

into 45,990 tons of CO2

emitted per year.

POWER SYSTEM STABILITY:

HVDC systems can never become

overloaded, and they offer

additional benefits through their

ability to control power flow and

voltage . HVDC can be very

effective in damping power

oscillations, as well as avoiding or

limiting cascading system

disturbances, particularly when

connecting two points inside the

same AC-grid, i.e., in parallel with

AC-lines: an HVDC Light®

converter is excellent at generating

or consuming reactive power.

Technical characteristics of

underground transmission system

When planning traditional

overhead transmission lines, it is

better to choose high voltage lines

for transmission over large

distances because not only can

transmission capacity be

increased but losses are also

reduced. However, for AC

transmission in underground cables

the situation is somewhat different.

If the voltage is increased, the

reactive power absorption of the www.1000projects.comwww.fullinterview.comwww.chetanasprojects.com

www.1000projects.comwww.fullinterview.comwww.chetanasprojects.comcable increases so that its technical

maximum length is reduced rather

than increased. The laws of physics

in this case then work against long

AC transmission. Today’s

experience of cable transmission

suggests a maximum transmission

distance of about 60 km for a

345 KV AC underground cable.

Reasons why underground HVDC

cables have a better environmental

profile than overhead HVAC lines

include land use, audible noise,

EMF ,material use, and power

SYSTEMS STABILITY.

HVDC Light, a new transmission

system designed for underground

transmission

This technology is based on some

key components: Extruded cable

technology Converter technology

Control and protection technology

Voltage source converters cause

less stress on the cables than

conventional HVDC converters and

this has enabled the development of

extruded cables

for HVDC. The extruded cable has

some significant advantages over

traditional mass impregnated

cables.

They are

1 Is completely oil free.

2. Has low weight.

3.Is very flexible and this simplifies

handling during installation.

4.Has very simple prefabricated

joints.

Voltage source converters also

show significant advantages over

traditional HVDC converters such

as:

Layout of HVDC transmission

1.Dramatically smaller size.

Typically they are half the height

and their footprint is 25 percent

smaller.

2.Superior voltage and reactive

power control reduces the risk of

blackouts.

3.They act as a “firewall” for

network disturbances and block the

cascadingwww.1000projects.comwww.fullinterview.comwww.chetanasprojects.com

www.1000projects.comwww.fullinterview.comwww.chetanasprojects.comtrips that occur in AC systems2).

4.They can operate in very weak

networks and do not require

network reinforcements.

5.They reduce down time after

outages with their “black start”

capability.

New high-speed control and

protection technology makes it

possible to fully utilize the inherent

benefits of the voltage source

converters.

TECHNICAL DEVELOPMENT

OF HVDC LIGHT SYSTEMS

HVDC Light technology was

introduced to the market in 1997

with a small test installation of 3

MW. Since then, both cables and

converters have progressed

dramatically in both size and

performance. Today the largest

system in service is a 330 MW

system operating at ± 150kV. A 350

MW system s currently under

construction. The converter design

has been refined by the adoption of

new switching schemes that reduce

the number of components and cut

the converter losses by 60%. In

contrast to traditional HVDC, an

HVDC Light system is highly

modularized and makes greater use

of semiconductors

The product matrix shown in

highlights available modules.

Increased environmental

pressure on overhead

transmission lines is both

raising total costs and

increasing the risk for

substantial project delays.

Cable installation techniques

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www.1000projects.comwww.fullinterview.comwww.chetanasprojects.comA crucial element in underground

transmission is the cable

installation technique. In the

Murraylink project

in Australia, and , a very successful

installation was implemented using

modified pipeline installation

equipment. Up to 3 km of cable was

successfully installed per day. The

total cost of laying the 170 km cable

system amounted to the very

reasonable sum of about AU$ 10

million ($ 7.6 million). HVDC Light

cables

have relatively low weight (typically

< 10 kg/m), making its installation

similar to that of fibre-optic cables:

the

equipment used for trenching and

the depth at which the cables are

laid is comparable (1 to 1.5 m below

the

surface).

Cost comparison Overhead lines –

Underground transmission The

new HVDC technology has, as

already mentioned, some unique

characteristics particularly when it

comes to increasing network

security. This means before a strict

cost comparison is performed, a

needs evaluation is required. Some

key checkpoints are Listed in below

table

TABLE:TABLE:

Need for power transmission

50–1000 MW

Need for accurate and fast

control

Distance more than 100 km

Difficult to obtain permits for

OHL

Asynchronous networks

Weak AC network

Risk for dynamic in Power

quality issues

Need for grid black start

capability

Need for high availability

although occurrence of

thunderstorms, wind

storms/hurricanes or heavily icing

conditions may apply

Need for low maintenance

Need for small footprint

Risk of low harmonic

resonances

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Need for fast voltage an

reactive power

control to enhance network

security

L

If at least three of these conditions are fulfilled it is likely that an HVDC Light system will offer a very attractive solution. If, however, OHL permits are difficult to obtain, then this reason alone is sufficient to warrant an HVDC Light® solution. In the following paragraphs, two examples currently being studied are presented.

Case 1,700 MW over 400 kmIt is assumed this case fulfils at least five of the criteria outlined in , such as:

The need for 50 –1000 MW. Transmission distance is

greaterthan 100 km. Difficulty to obtain permits

for OHL. Risk of dynamic instability. Need for fast voltage and

reactive power control to enhance

networksecurity.A comparison of the direct investmentcost shows the following span:The direct investment cost for HVDC Light option including converters, cables and their installation is in the range of $ 275 – $ 420 million. Thebreadth of this range is primarily due to differences in installation costs and local market conditions. For the AC overhead option there is an even

greater span in cost. A study made by ICF consultancy in 2001shows a huge variation in costs from country to country. Using these data,the direct investment cost for the AC overhead option gives a cost range of $ 130 – $ 440 million for the line including installation and substations. At the direct investment cost levelthe price for the underground alternative is between 0.6 and 3.2 times the overhead option. This is quite adifference from the normally anticipated 5 – 15 times. Furthermore, other factors should also be considered,for example:

Additional investments in equipment to manage voltage and reactive power control in the AC case.

Losses (both cases). Costs for permitting the

overhead solution. Cost for permission and

construction time (both cases).

Increased transmission capacity in the existing AC grid (HVDC case).

Loss of property value.When these factors are included in the evaluation, the competitiveness of the HVDC alternative increases. Assume, for example, the following realistic additional factors for the overhead option:

Additional reactive compensation $25millions

Loss of property value: $ 25 million.

Value of increased transmission capacity in existing AC grid

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www.1000projects.comwww.fullinterview.comwww.chetanasprojects.comConclusionsIncreased environmental pressure on overhead transmission lines is both raising total costs and increasing the risk for substantial project delays. New HVDC technology in the form of HVDC Light® has made underground options technically feasible and economically viable. This is especially so if the new grid investment is drivenby security of supply issues. The conventional view that an underground link will cost 5 – 15 times its overhead counterpart must be revised. Depending on local conditions, it is realistic that the costs for an underground high-voltage line are equal to that oftraditional overhead lines.

HVDC Light converter technology

"The Pulse Width Modulated Voltage Source

Converter a close to ideal component in the

transmission network. From a system point

of view it acts as a motor or generator

without mass that can control active and

reactive power almost instantaneously."

Conventional HVDC converter technology is

based on the use of line-commutated or

phase-commutated converters (PCC). With

the appearance of high switching frequency

components, such as IGBTs (Insulated Gate

Bipolar Transistor) it becomes advantageous

to build VSC (Voltage Source Converters)

using PWM (Pulse Width Modulation)

Technology .

The key part of the HVDC Light

converter consists of an IGBT valve bridge.

No special converter transformers are

necessary between the valve bridge and the

AC-grid. A converter reactor can separate

the fundamental frequency from the raw

PWM waveform. If the desired DC voltage

does not match the AC system voltage, a

normal AC transformer may be used in

addition to the reactor. A small shunt AC-

filter is placed on the AC-side of the reactor.

On the DC-side there is a DC capacitor

that serves as a DC filter too.

Active and reactive power control

The fundamental frequency voltage across

the converter reactor defines the power flow

between the AC and DC sides. Changing the

phase angle between the fundamental

frequency voltage generated by the converter

(Ug) and the voltage on the AC bus controls

the active power flow between the converter

and the network. The reactive power flow is

determined by the amplitude of Ug, which is

controlled by the width of the pulses from the

converter bridge.

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The control is performed by the MACH

2™ system developed by ABB. All functions

for control, supervision and protection of the

stations are implemented in software running

in a family of microprocessor circuit boards.

The IGBT is characterized by:

Multi-chip design (chip paralleling is easy)

Forward blocking only7 Current limiting characteristics

Fully controllable turn-off

4. UNDERGROUNDING AND RIGHT-OF-WAYEntering an urban or sub-urban area with a new high-voltage overhead line may in some cases be very difficult or even impos- -sible due to scarcity of available land. Then some of the alternatives may be increased generation capacity in the city, installing AC-cables or installing DC-cables. Both the alternatives with AC and DC cables can be motivated and are technically feasible. In the case that the DC alternative is chosen for reasons of system benefits, it is most suitable to make use of polymer DC cables, as for example the HVDC-Light® cable system,instead of their paper/oil insulated counterparts. Installation of polymer HVDC cable systems is easier for cityinfeed purposes due to the flexibility and robustness of the cables and the quick installation of joints [10].Jointing polymer HVDC cables takes considerably less time than jointing paper/oil insulated ones.HVDC cables generate a static magnetic field, just as our own mother earth. The magnitude of the magnetic field

generated by a pair of HVDC cables in typical installation conditions is far lower than the earth magnetic fieldwe all are exposed to. Therefore, no harmful effects worse than those induced by our own earth are induced bythe cable system. A typical graph is shown in is compared Figure 3, where the earth magnetic field is compared

AC-cables generally solve the problem of right-of-way but may be difficult to implement due to reactive powerbalancing problems, added short circuit power and in some cases limited transmission distance [4]. An AC-cablenetwork has a large generation of reactive power during low-load, forcing measures such as shunt reactors to beimplemented.The DC cable alternative gives no technical limit to the transmission distance, adds no short circuit power andactually enables improved reactive power balance due to the properties of the converters. The power handling ofa given cable dimension is higher for DC than for AC due to better utilization of the insulation and lower lossesin conductor and shield.Figure 4. Pair of extruded land DC cables showing from center outward: aluminum conductor,conducting XLPE- layer, dc-insulation XLPE, ground side conductive XLPE, copper shield wires, aluminumouter wrapping and outer sealing and mechanical protection..Today, cables for VSC based HVDC are manufactured with extruded plastic insulation, meaning that they areoil-free by design. The extruded cables are more flexible than their oil-impregnated counterparts and they aremuch more easily jointed using pre-fabricated joints.

What is HVDC Light?HVDC > HVDC Light > What is HVDC Light?

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HVDC Light® is the successful and environmentally-friendly way to design a power transmission system for a submarine cable, an underground cable or network interconnection.

HVDC Light® is HVDC technology based on voltage source converters (VSCs). With extruded DC cables, power ratings from a few tens of megawatts up to several hundreds of megawatts are available.

..

HVDC Light® cables have extruded polymer insulation. Their strength and flexibility make the HVDC Light® cables well suited for severe installation conditions both underground as a land cable and as a submarine cable.

The converter station design is based on voltage source converters (VSCs) employing state of the art turn-on/turn-off IGBT power semiconductors that operate with high frequency pulse width modulation.

HVDC Light® has the capability to rapidly control both active and

reactive power independently of each other, to keep the voltage and frequency stable. This gives total flexibility regarding the location of the converters in the AC system since the requirements of short-circuit capacity of connected AC network is low (SCR down to zero).

converter stations are also small. All equipment except the power transformers is indoors. Well-proven and tested equipment at the factory make installation and commissioning quick and efficient.

The stations are designed to be unmanned. They can be operated remotely or could even be automatic, based on the needs of the interconnected AC networks. Maintenance requirements are determined mainly by conventional equipment such as the AC breakers, cooling system, etc.

A pair of HVDC Light® cables

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The submarine cables can be laid in deeper waters and on rough bottoms.

The land cables can be installed less costly with ploughing technique.

HVDC cables can now also go overhead with aerial cables.

The environmental benefits is:

Magnetic fields are eliminated since HVDC Light® cables are laid in pairs with DC currents in opposite directions.

Risk of oil spill, as in paper-oil-insulated cables, is eliminated.

The cable insulation is PE based and not dangerous.

The cable metals can be reused.

Power transmission via cables gives

no visual impact no ground current no electromagnetical fields.

HVDC Light® is an alternative to conventional AC transmission or local generation in many situations. Possible applications include:

Connecting wind farms Underground power links Powering islands Oil & gas offshore

platforms; power from shore

Asynchronous grid connection

City centre i HVDC Light cables from ABBReferencesHvdc Power Transmission Systems: Technology and System Interactions:KR PadiyarHVDC Facts and controllers:VK SoodA text book of electrical power :J.B.Gupta

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